A standard split-jaw lathe chuck has a chuck body rotatable bout a chuck axis and formed with a plurality of angularly spaced and radially extending inner guides receiving respective inner jaws parts, and with a plurality of angularly spaced and radially extending outer guides receiving respective outer jaw parts projecting axially from the chuck body. Respective coupling members are axially displaceable in the inner jaw parts between axially advanced coupling positions radially linking the respective jaw parts and axially retracted decoupling position permitting relative radial displacement of the respective jaw parts. These coupling members may be jointly displaceable as described in U.S. Pat. No. 3,682,491, or independently displaceable as described in my earlier copending application Ser. No. 886,348 filed Mar. 14, 1978.
The advantage of such a structure is that it allows the outer jaw parts to be roughly adjusted for the workpiece to be clamped, so that only a modest radial stroke is necessary for the jaws to clamp such a workpiece. As a result a large mechanical advantage can be used so that the operating member moves through a relatively long axial stroke to produce a relatively short radial stroke of the jaws. Hence a good mechanical advantage is obtained for extremely tight gripping of a workpiece. What is more exchanging of the jaws, or using jaw blanks such as described in my jointly filed and copending application is also possible.
One of the recurring problems with such a chuck is that when spun during a machining operation the radially outwardly effective centrifugal forces on the jaws loosen the hold of the chuck on the workpiece. Thus it is known to provide force compensation such as described in my copending and jointly filed application. In such an arrangement radially displaceable weights on the chuck body are engaged via force-converting members with the inner jaw parts so that radially outwardly effective forces in these weights are converted into radially inwardly effective forces applied to the inner jaw parts. In this manner centrifugal forces can be balanced in the chuck.
These centrifugal forces are advantageously employed in split-jaw lathe chucks of the type described in my above-cited copending application Ser. No. 886,348 wherein radially displaceable cams are provided in each of the inner jaw parts engaging the respective coupling member. These cams are radially displaceable between outer positions urging the coupling members forwardly into the coupling positions and radial inner positions allowing the coupling members to move axially backwardly. Thus in such a system the radially outwardly effective forces on these cams ensure that as rotation speed increases the forces pressing the coupling members into engagement with the respective outer jaw parts also increase.
A considerable disadvantage of this last-described type of chuck is that it is necessary to act on the cams directly at each of the jaws to separate the respective inner and outer jaw parts. This means that it is frequently necessary for the lathe operator to rotate the chuck while setting the jaws so as to have access to the various cams. Obviously in some machining situations this is extremely difficult, as the workpiece being mounted in the chuck must similarly be rotated.